This invention relates to a spherical lens having an uneven refractive index and a construction capable of correcting spherical aberration.
In optical communication and optical recording-regenerating techniques in the optoelectronics field, which have made remarkable technological progress particularly in recent years, improvements in various apparatuses including luminescent elements, transmission means such as optical fibers, and information processing units, have advanced rapidly, attaining higher performance and reliability that have readied these apparatus for practical use. However, the optical systems constituting one part of such apparatuses are still much indebted to conventional technology and therefore, it is strongly desired to have high performance optical systems in order to allow said main apparatuses to fully exhibit their excellent performance.
For example, when information signals are read out of an optically recorded PCM disk, a light beam is focused on a recording surface on which information is recorded in the form of pits so that the information is read out by detecting changes in the intensity of the light reflected by the recording surface. This type of optical pickup will be miniaturized in line with the trend toward miniaturization of overall systems. Hence it is considered that the focal length of the lens should be made as short as possible, and for this, a spherical lens having a large refractive index is considered useful.
On the other hand, in optical communication it is common to optically connect optical fibers by interposing a spherical lens between them so as to allow beams emitted from one fiber to enter the other fiber through its end surface. Also in this case it is required that the beams emitted from the fiber be accurately focused on the end surface of the other fiber. Further, spherical lenses with short focal lengths are advantageously employed not only in such microelectronics field as image processing and phase-information processing, but also for the optical systems of medical laser scalpels and microcameras.
The spherical lens, however, suffers the disadvantage of having large spherical aberration. This is true not only of spherical lenses having a uniform refractive index throughout but even of those having a refractive index which has been gradually reduced from the center toward the periphery by ion exchange since in the latter case it is not possible to obtain a sharp enough refractive index gradient. As illustrated in FIG. 1(A), the paraxial rays from a point O on the optical axis on one side of a spherical lens having a continuously graded refractive index from the center to the periphery intersect the optical axis at a point Po on the other side of the lens, while emerging rays having a larger exit angle (.theta.) intersect the optical axis at a point Pa which is nearer to the lens 1 than the point Po. Assuming that the intersection point Po of the paraxial rays with the optical axis is on an image plane S, lateral aberration is represented by the distance .DELTA.t between the point Po and the intersection point at which the emerging rays having a larger exit angle intersect the image plane S. Usually, as shown in FIG. 1(B), in the spherical lens, the larger the exit angle .theta. of the ray is, the larger the lateral aberration .DELTA.t becomes. This is inevitably true not only of spherical lenses, but also of lenses in general. Even a spherical lens improved by being given a graded refractive index as shown in the graph of FIG. 1(A) still has large aberration because the slope of the refractive index gradient cannot be made large.
Such spherical aberration has been an obstacle to simplification of the optical pickup in optical recording-regenerating apparatuses and to good optical coupling in optical communication.
Spherical lenses of this type having a refractive index graded from the center thereof towards the periphery are known Maxwell fisheye lenses and Luneburg lenses. The Maxwell fisheye lens is capable of focusing a ray of light originating from a given point on the spherical surface at the symmetrically opposite point on the spherical surface but cannot emit a ray of light to the outside of the spherical lens. The latter lens focuses a parallel incident light flux received on one side of the lens at a point on the other side on the spherical surface, but cannot focus the flux outside the spherical lens.
Morgan has proposed an ideal refractive index distribution for a generalized Luneburg lens which would allow a light flux from a point exterior of the lens to be focused at a point on the opposite side of the lens without aberration. However, the technology for producing such a lens has not been developed. Assume that the gradient of refractive index is to be given to the lens by the ion exhange method. While a refractive index which decreases approximately in proportion to the square of the distance from the center can be attained by the ion exchange method, this method cannot produce the ideal refractive index gradient according to Morgan. The refractive index gradient in the Maxwell fisheye lens and the Luneburg lens is so steep as to make production of such lenses even more difficult.
Because of the problems inherent in conventional spherical lenses, there is needed a lens which is simple in construction and easy to produce and which has little aberration and can be easily applied to various apparatuses.